Conventionally, a low pressure CVD apparatus has been widely utilized, particularly for forming a Si.sub.3 N.sub.4 film, a polysilicon film, and so on, when a semiconductor device is manufactured.
Of the known low pressure CVD devices, a vertical batch type CVD apparatus is currently most popular. However, since wafers are expected to have increasingly larger diameters in future and a short TAT (Turn Around Time) is required for reducing a development term. A single wafer type CVD apparatus (for example, a single-wafer type reduced pressure CVD apparatus) has been developed for replacement of the vertical batch type CVD apparatus and has been gradually introduced into a portion of semiconductor device developing lines.
The single-wafer type reduced pressure CVD apparatus may be generally classified by two types of heating methods: a resistive heating hot wall type and a lamp heating cold wall type.
Within these classes, a single wafer type low pressure CVD apparatus of resistive heating hot wall type is similar to the conventional vertical type low pressure CVD apparatus in process conditions because the vertical low pressure apparatus also belongs to the resistive heating type, so that it can be readily replaced with the conventional apparatus. Moreover, since the resistive heating hot wall type has an additional feature of exhibiting a good temperature controllability, it is regarded as being advantageous over the lamp heating cold wall type.
As such a single wafer type reduced pressure CVD apparatus of resistive heating hot wall type, one capable of simultaneously processing two wafers, for example, silicon wafers for achieving higher through-put is also available partially in the market.
FIGS. 2A and 2B are a side view and a plan view showing an example of a heat processing chamber for a resistive heating hot wall type single-wafer type reduced pressure CVD apparatus, as mentioned above, which is capable of simultaneously processing two wafers.
This apparatus is provided with a heat processing chamber 1 serving as an reaction chamber for carrying out a film formation processing. A heater (not shown) is arranged outside the heat processing chamber in contact with a upper wall surface and a lower wall surface thereof which allow for adjustment to a desired film formation processing temperature. The heat processing chamber 1 is also provided inside thereof with a wafer carrier 2. The wafer carrier 2 is composed of an upper stage 2a and a lower stage 2b, as illustrated in FIG. 2A, each of the upper stage 2a and the lower stage 2b is constructed of legs 3 and a carrier plate 4. A circular opening 5, sufficiently larger than a wafer W as illustrated in FIG. 2B, is formed through each carrier plate 4 of the respective upper stage 2a and lower stage 2b, and a pin 6 protruding into the opening 5 is formed on the circumferential edge of the opening 5 at four diametrically opposing positions. These pins 6, which are made of a quartz boat and arranged below the top surface of the carrier plate 4, are formed such that the spacing between the opposing pins 6, 6 is smaller than the diameter of the wafer W to support the wafer W at their tip ends.
In order to subject the wafer W to film formation processing by using a low pressure CVD apparatus having the heat processing chamber 1 constructed as described above, conventionally, the heat processing chamber 1 is heated at first by the heaters to a film formation processing temperature. Then, two wafers W are carried on transporting arms (not shown), respectively which are then inserted into an accommodation port 7 of the heat processing chamber 1, and the wafers W are immediately mounted on the pins 6 of the upper stage 2a and on the pins 6 of the lower stage 2b, respectively, in a bridging style for achieving higher through-put.
Subsequently, when the wafer W is heated to the temperature within the heat processing chamber 1, i.e., the film formation processing temperature, a material gas is introduced into the inside of the heat processing chamber 1 from a material gas introducing tube, not shown, to perform the film formation processing.
In the film forming method as described above, however, when the temperature within the heat processing chamber 1 reaches 700-850.degree. C., many particles of foreign substances are observed on the top surface, i.e., the film forming surface of the wafer W mounted on the lower stage 2b. The occurrence of such foreign substances causes a reduced yield.
The present invention has been made in view of the problem mentioned above, and its object is to provide a method of manufacturing a semiconductor device which prevents foreign substances from being produced on a wafer.